Phosphorus mobilization in agroforestry: Organic anions, phosphatase activity and phosphorous fractions in the rhizosphere

S. Radersma, Pauline Grierson

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    Abstract

    In agroforestry systems on Ferralsols in the tropics, maize crop yields are low owing to a lack of P. However, someagroforestry tree species adapted to P-fixing soils may be able to utilize less available P sources and concurrentlyincrease P availability to adjacent crops. Adaptations for enhanced P acquisition from P-fixing soils include exudationof low molecular weight (LMW) organic anions (OA) and phosphatase enzymes.We identified major organicanions, and measured acid phosphatase activity and P fractions in the rhizosphere soil of maize (Zea mays L.), aswell as in the perennial species Grevillea robusta A. Cunn., Cassia spectabilis DC. (syn. Senna spectabilis (DC.)H.S. Irwin and R.C. Barneby), Tithonia diversifolia (Hemsl.) A. Gray, Eucalyptus grandis W. Hill ex Maiden andCedrella serrata Royle. Maize and trees were grown simultaneously at field sites and in large pots. Rhizospheresoil of C. spectabilis contained at least 29 μmol oxalate g−1 soil, derived from a high exudation rate in the rangeof at least 5–10 μmol oxalate m−1 root day−1. Incubation of Ferralsols with much lower concentrations of citrateincreased labile P fractions, but there was no clear relation between OA concentration and an increase in labileP fractions in rhizosphere soils, where P mobilization and P uptake occur at the same time. Acid phosphataseactivity in rhizosphere soil of all species was two to five times greater in rhizosphere soil compared with bulk soil,and correlated in rhizosphere soil of G. robusta with a shift from organic P to inorganic P in soil P fractions. Weconclude that organic anion exudation and acid phosphatase activity of tree roots may increase mobilization of Pin the rhizosphere, the extent of which depends on the species, the organic anion and pH. However, it is unlikelythat the extent of P mobilization will benefit adjacent crop plants unless crop roots exert insufficient P-mobilizationeffects themselves, and grow in the rhizosphere of tree roots.
    Original languageEnglish
    Pages (from-to)209-219
    JournalPlant and Soil
    Volume259
    Issue number1/2
    DOIs
    Publication statusPublished - 2004

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